Geologic Background:
Guagua Pichincha and the older Pleistocene Rucu
Pichincha stratovolcanoes rise immediately W of Quito at
the west end of the 25-km-long volcanic complex. The
horseshoe-shaped summit crater, ~2 km in diameter and 600
m deep, was breached to the west during a
late-Pleistocene slope failure ~50,000 years ago.
Subsequent late-Pleistocene and Holocene eruptions from
the central vent consisted of explosive activity with
pyroclastic flows accompanied by periodic lava dome
growth and destruction. The volcano has no permanent ice
cap.
Historic Activity:
- Many minor eruptions have occurred since the Spanish
era.
- The central lava dome was probably emplaced during
the volcano's largest historical eruption, in 1660, that
dropped 30 cm of ash on Quito and generated W-flank
pyroclastic flows.
- Phreatic activity in 1981 and 1982 ejected a small
amount of ash and blocks.
- A single phreatic explosion occurred from the same
area in 1985. The phreatic vents were on the NE flank of
the central lava dome.
Recent Activity:
- Local seismicity has increased substantially since
mid-April 1988, accompanied by minor changes in the
summit crater's fumarole field. Seismicity gradually
built from background levels of 5-10 per month to more
than 200 per month by August, and was continuing in early
October. August and September events were centered a 4-9
km depth near and N of the s caldera wall.
- In late September and early October 1988 seismicity
migrated N toward the modern vent and upward, with some
foci as shallow as 1.5 km.
- After 15 months of quiet, phreatic activity began on
April 16, 1990.
- A phreatic explosion on March 12, 1993 in the young
crater of Guagua Pichincha's central dome killed 2 young
volcanologists.
- Series of phreatic explosions during 1997.
- Phreatic explosions from August 1998 to June 1999 and
rising earthquakes. Explosions and steam and ash plumes
in September with growth of a new dome beginning in
October and continuing through January 2000.
- Minor ash eruptions through July 2000 with partial
collapse of the January dome on July 12.
- In 2001, gradual dome growth through March and ash
emissions through May.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 6, No. 9 to V. 13,
No. 9)
- Global Volcanism Network (V. 15, No. 3 to V. 26, N. 9
).
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